Testing
Got word just now that the test runs were good and the engine will be shipped out at 1500 today to the Pilatus service center (SC) with one of their rigging techs.
I forget to share a new concept I learned in this process...DOD or Domestic Object Damage as opposed to the more commonly used FOD or Foreign Object Damage. First the test stand photos.
The test stands techs monitoring the test run with my engine in the background...
A close up of some of the data they collect. Note this screen is just one of 14 tabs with various parameters. This one is called "trim" which (I think) is the data they use to calibrate (they call it trimming) the ITT probes.
Now about DOD.
As I mentioned in a previous post, the only unusual item in my overhaul was the need to replace 10 CT (compressor turbine) blades. The compressor turbine is in the "hot" part of the gas generator section (many people call it the compressor) that drives the centrifugal and axial flow compressors in the "cool" part of the engine that squeezes the ambient air and pushes it into the hot section.
Note that the CT and PTs (power turbines) are not physically connected. That is why a PT6 is called a "free turbine" and why it is so easy to spin the prop by hand...you aren't turning the whole engine, just the two PTs (in the case of my engine, smaller PT6s have one PT).
Note that the CT and PTs (power turbines) are not physically connected. That is why a PT6 is called a "free turbine" and why it is so easy to spin the prop by hand...you aren't turning the whole engine, just the two PTs (in the case of my engine, smaller PT6s have one PT).
For reference, here is what the complete CT looks like (not from my engine)...
I am told it is common to replace CT blades on a second overhaul, but not typical on a first run overhaul. These ten blades showed evidence of cracks outside the overhaul manual limits and thus it was required to replace them. Here are some photos which don't show much to me, but they had two experts and a PWC engineer look at them and all came to the same conclusion...replacement was needed.
Naturally, I wanted to understand better how this damage occurred. These little guys are very expensive (and there are about 45 on the CT), but more importantly I wanted to know if I needed to change something in how I operate the plane to prevent such damage.
Naturally, I wanted to understand better how this damage occurred. These little guys are very expensive (and there are about 45 on the CT), but more importantly I wanted to know if I needed to change something in how I operate the plane to prevent such damage.
Turns out they said the damage was not FOD (foreign object damage) since any FOD would have had to work its way through the centrifugal and axial flow compressors which were absolutely clean. However, they could tell it was impact damage of some sort. Thus, the conclusion was it must have been domestic object damage (DOD). Examples of that include small pieces of carbon or ceramic coating loss. Or, It could even be a specific batch of CT blades that had a different composition and were more prone to some very minor cracking over time in normal operation.
Most importantly to me, they said there definitely was nothing indicating hot section distress or operational issues. The hot section was very clean (other than these blades) according to the technicians. But, the overhaul manual does not allow any cracks at all in this area (leading edge) of the CT blades, so they get replaced. Interestingly (to me), they said they do not do this level of inspection at an HSI (hot section inspection) and thus these cracks may have been there before my HSI, but undetected.
Also interesting to me is if they cannot come to the conclusion that this was impact damage (or otherwise explain the damage), they have to replace all the CT blades. So, I guess I am glad they could identify this as DOD and not something unexplained.
Guess this is why you do overhauls...really look at each component in detail. With a turbine wheel spinning at 36,000 RPM, I am 100% OK with taking a conservative approach to these sorts of things. If a piece of metal lets loose at 36,000 RPM, it can do a lot of damage quickly.
They also replaced two first stage PT (power turbine) blades for the same reason, but they said that was typical. I had the entire second stage PT replaced (43 blades) as it has a 5,000 hour life limit and I did not want to tear the engine down again in a few years (my engine has 3,500 hours on it now). PWC offers a "campaign" (discount) of 50% off the list price to encourage this second stage PT replacement. As I mentioned earlier, this exact same part has a 12,000 hour life limit on twins (like the King Air), but they are super conservative when it comes to singles.
Looking forward to having the bird back in my hands soon.